Teritary oil recovery combined with gas conversion process

ABSTRACT

A process for the recovery of oil from a subsurface reservoir in combination with the production of liquid hydrocarbons from a hydrocarbonaceous stream involving:  
     (i) separating an oxygen/nitrogen mixture into a stream enriched in oxygen and an oxygen depleted stream;  
     (ii) partial oxidation of the hydrocarbonaceous feed at elevated temperature and pressure using enriched oxygen produced in step (i) to produce synthesis gas;  
     (iii) converting synthesis gas obtained in step (ii) into liquid hydrocarbons; and  
     (iv) recovering oil from a subsurface reservoir using at least part of the oxygen depleted gas stream produced in step (i).

[0001] The present invention relates to a process for the enhancedrecovery of oil from a subsurface reservoir in combination with theproduction of liquid hydrocarbons from a hydrocarbonaceous stream.

[0002] A first element of the present invention concerns the enhancedrecovery of oil from a subsurface reservoir.

[0003] Enhanced oil recovery (sometimes also called tertiary oilrecovery) is the description applied by the oil industry tonon-conventional techniques for getting more oil out of subsurfacereservoirs than is possible by natural production mechanisms (primaryoil recovery) or by the injection of water or gas (secondary oilrecovery).

[0004] If oil is to move through the reservoir rock to a well, thepressure under which the oil exists in the reservoir must be greaterthan that at the well bottom. The rate at which the oil moves towardsthe well depends on a number of features, among which the pressuredifferential between the reservoir and the well, permeability of therock, layer thickness and the viscosity of the oil. The initialreservoir pressure is usually high enough to lift the oil from theproducing wells to the surface, but as the oil is produced, the pressuredecreases and the production rate starts to decline. Production,although declining, can be maintained for a time by naturally occurringprocesses such as expansion of the gas in a gas cap, gas release by theoil and/or the influx of water. A more extensive description of naturalproduction mechanisms can be found in the Petroleum Handbook, 6^(th)edition, Elsevier, Amsterdam/New York, 1983, p. 91-97.

[0005] The oil not producible, or left behind, by the conventional,natural recovery methods may be too viscous or too difficult to displaceor may be trapped by capillary forces. Depending on the type of oil, thenature of the reservoir and the location of the wells, the recoveryfactor (the percentage of oil initially contained in a reservoir thatcan be produced by natural production mechanisms) can vary from a fewpercent to about 35 percent. Worldwide, primary recovery is estimated toproduce on average some 25 percent of the oil initially in place.

[0006] In order to increase the oil production by natural productionmechanisms, techniques have been developed for maintaining reservoirpressure. By such techniques (also known as secondary recovery) thereservoir's natural energy and displacing mechanism which is responsiblefor primary production, is supplemented by the injection of water orgas. However, the injected fluid (water or gas) does not displace allthe oil. An appreciable amount remains trapped by capillary forces inthe pores of the reservoir rock and is bypassed. This entrapped oil isknown as residual oil, and it can occupy from 20 to 50 percent, or evenmore, of the pore volume. See for a more general description ofsecondary recovery techniques the above-mentioned Petroleum Handbook, p.94-96.

[0007] Enhanced oil recovery (sometimes called tertiary oil recovery) isthe description applied by the oil industry to non-conventionaltechniques for getting more oil out of subsurface reservoirs than ispossible by natural production mechanisms or secondary productionmechanisms. Many enhanced oil recovery techniques are known. It coverstechniques as thermal processes, miscible processes and chemicalprocesses. Examples are heat generation, heat transfer, steam drive,steam soak, polymer flooding, surfactant flooding, the use ofhydrocarbon solvents, high-pressure hydrocarbon gas, carbon dioxide andnitrogen. See for a more general description of secondary recoverytechniques the above-mentioned Petroleum Handbook, p. 97-110.

[0008] The use of nitrogen in enhanced oil recovery processes is wellknown. At first waste gases as stack gas, flue gas and exhaust gas wereused. These gasses usually contained not only nitrogen, but also carbondioxide and optionally steam. See for instance U.S. Pat. No. 4,499,946.A problem, however, was the presence of certain waste products such asnitrogen oxides and sulphur oxides which give rise to corrosion andpollution problems. A paper by M. D. Rushing et al., entitled “MiscibleDisplacement with Nitrogen”, Petroleum Engineer, November 1977, p.26-30, describes a miscible oil displacement process involving theinjection of high pressure nitrogen. As disclosed, pure nitrogen isinjected into the reservoir and functions to initially strip relativelylow molecular weight hydrocarbons from the reservoir oil. U.S. Pat. No.4,434,852 describes the use of nitrogen and 2 to 20 percent by volume oflight hydrocarbons in the enhanced oil recovery of subterranean oilreservoirs. Mixtures of nitrogen and carbon dioxide are described inU.S. Pat. Nos. 3,811,501 and 4,008,764. The injected gas may take theform of substantially pure nitrogen, such as produced by cryogenicfractionation of air as described by Rothrock et al., Nitrogen FloodsNeed Specialise Surface Equipment, Petroleum Engineer, August 1977, p.22-26. As described above, the nitrogen gas may also take the form offlue gasses such as from boilers or internal combustion engines whichtypically will contain about 80-90% nitrogen, usually 88%, 5-15% carbondioxide, usually 10%, 0-2% carbon monoxide, usually 1%, and theremainder hydrogen and trace amounts of other gasses.

[0009] As described, attention has been given to producing nitrogencryogenically. A problem, however, is the need of a large, expensivecryogenic unit.

[0010] Several cryogenic concepts have been developed over the years toliquefy and separate air into its main constituents nitrogen, oxygen andrare gases. Refrigeration for cryogenic applications is produced byabsorbing or extracting heat at low temperature and rejecting it to theatmosphere at higher temperatures. Three general methods for producingcryogenic refrigeration in large-scale commercial application are theliquid vaporisation cycle, the Joule-Thomson expansion cycle and theengine expansion cycle. The first two are similar in that they bothutilise irreversible isenthalpic expansion of a fluid, usually through avalve. Expansion in an engine approaches reversible. isenthalpicexpansion with the performance of work. For more detailed discussionreference is made to Perry's Chemical Engineers Handbook, Sixth Edition,12-49ff. (McGraw-Hill, New York, 1984), Kirk-Othmer, Encyclopedia ofChemical Technology, Fourth Edition, Volume 7, p. 662 ff. (John Wileyand Sons, New York, 1993) and Ullmann's Encyclopedia of IndustrialChemistry, Fifth Edition, Volume A 18, p. 332 ff. (VCH, Weinheim, 1991).

[0011] Most commercial air separation plants are based on Linde's doubledistillation column process. This process is clearly described in theabove references. In a typical example, feed air is filtered andcompressed to a pressure usually between 5 and 10 bara. The compressedair is cooled and any condensed water is removed in a separator. Toavoid freezing of water and carbon dioxide in the cryogenic part of theplant, the feed air is further passed through an adsorbent bed, usuallyactivated alumina and/or molecular sieves, to remove the last traces ofwater and carbon dioxide. The purified air is than cooled down further,and fed to a first cryogenic distillation unit, usually at anintermediate stage. Crude liquid material from the bottom section of thefirst distillation unit, usually comprising between 40 and 50 molpercent oxygen, is fed to the second distillation unit (which secondunit is usually on the top of the first distillation unit, the condenserof the first column usually acting as the reboiler for the second unit),usually also at an intermediate stage. The second distillation unit isoperated at relatively low pressure (usually 1 to 2 bara). At the top ofthe first distillation unit almost pure liquid nitrogen is obtainedwhich is typically fed to the second column at the top. Pure liquidoxygen is obtained at the bottom of the second distillation unit, whilepure gaseous nitrogen is obtained from the top of the second column.

[0012] Many variations on the above concept are known. These includeseparation of air into gaseous products, liquid products and all kind ofcombinations thereof. Also the production of partly enriched oxygenand/or nitrogen streams together with almost pure oxygen and/or nitrogenstreams, either in liquid or gaseous phase is well known. In additionthere may be additional distillation units to separate any of the raregases present in the feed air. Further, the methods for creating the lowtemperatures may vary in many ways. In this respect reference is made tothe above cited literature references, and further to EP 798524, JP08094245, EP 593703, EP 562893, U.S. Pat. No. 5237822, JP 02052980, EP211957, EP 102190, SU 947595 JP 71020126 and JP 71020125.

[0013] A second element of the present invention concerns the productionof liquid hydrocarbons from a hydro-carbonaceous feed.

[0014] Many publications are known describing processes for theconversion of (gaseous) hydrocarbonaceous feed stocks, as methane,natural gas and/or associated gas, into liquid products, especiallymethanol and liquid hydrocarbons, particularly paraffinic hydrocarbons.In this respect often reference is made to remote locations (e.g. in thedessert, tropical rairi-forest) and/or offshore locations, where nodirect use of the gas is possible, usually due to the absence of largepopulations and/or the absence of any industry. Transportation of thegas, e.g. through a pipeline or in the form of liquefied natural gas,requires extremely high capital expenditure or is simply not practical.This holds even more in the case of relatively small gas productionrates and/or fields. Reinjection of gas will add to the costs of oilproduction, and may, in the case of associated gas, result in undesiredeffects on the crude oil production. Burning of associated gas hasbecome an undesired option in view of depletion of hydrocarbon sourcesand air pollution. Gas found together with crude oil is known asassociated gas, whereas gas found separate from crude oil is known asnatural gas or non-associated gas. Associated gas may be found as“solution gas” dissolved within the crude oil, and/or as “gas cap gas”adjacent to the main layer of crude oil. Associated gas is usually muchricher in the larger hydrocarbon molecules (ethane, propane, butane)than non-associated gas.

[0015] In WO 91/15446 a process is described to convert natural gas,particularly remote location natural gas (including associated gas),into liquid hydrocarbons suitable for use as fuel. However, no optimallyintegrated, efficient, low-cost process scheme has been described.

[0016] In WO 97/12118 a method and system for the treatment of a wellstream from an offshore oil and gas field has been described. Naturalgas is converted into syngas using pure oxygen in an autothermalreformer, a combination of partial oxidation and adiabatic steamreforming. The syngas (comprising a considerable amount of carbondioxide) is converted into liquid hydrocarbons and wax. No fully andoptimally integrated process scheme for a highly efficient, low capitalprocess is described in this document.

[0017] In EP 1 004 746 a process has been described for the combinedproduction of liquid hydrocarbons and the recovery of oil from asubsurface reservoir by partial oxidation of natural gas followed byconversion of the synthesis gas thus obtained into hydrocarbons andseparating the hydrocarbons into liquid hydrocarbons and gaseoushydrocarbons (mainly C₁-C₄ hydrocarbons), and combusting and/orexpanding these gaseous hydrocarbons to provide power for the secondaryor enhanced recovery of oil. However, a further optimisation of theefficiency and the integration of the process is desired.

[0018] An object of the present invention is to provide furtherimprovements for an efficient, low cost, process-and energy-integratedprocess scheme for the production of (easily manageable) liquid andsolid hydrocarbons from light hydrocarbons. The further improvementconcerns the combined production of hydrocarbons and the enhancedrecovery of oil from subsurface reservoirs. It has been found that anefficient process could be developed by separating an oxygen/nitrogenmixture, especially air, into an oxygen rich stream and an enrichednitrogen stream, preferably a pure, i.e. at least 98 vol % pure,nitrogen stream. The oxygen rich stream can be used for the partialoxidation of the natural gas, while the nitrogen stream can be used forenhanced oil recovery. It is observed in this respect that hydrocarbonssynthesis starting from synthesis gas made from a hydrocarbonaceous feedand oxygen made by an air separation unit is well known. However, uptill now it has not been realised that the nitrogen, made in the sameprocess, could be used in the enhanced oil recovery. In this respect itis observed that several processes are known for the production ofsynthesis gas which do not use pure oxygen, such as steam methanereforming and (partly) autothermal reforming. Further, also air may beused in the preparation of synthesis gas. Thus, a number of options forthe preparation of synthesis gas for use in the Fischer-Tropschsynthesis are available.

[0019] The present invention relates to a process for the recovery ofoil from a subsurface reservoir in combination with the production ofliquid hydrocarbons from a hydrocarbonaceous stream, comprising:

[0020] (i) separating an oxygen/nitrogen mixture into a stream enrichedin oxygen and an oxygen depleted stream;

[0021] (ii) partial oxidation of the hydrocarbonaceous feed at elevatedtemperature and pressure using enriched oxygen produced in step (i) toproduce synthesis gas;

[0022] (iii) converting synthesis gas obtained in step (ii) into liquidhydrocarbons;

[0023] (iv) recovering oil from a subsurface reservoir using at leastpart of the oxygen depleted gas stream produced in step (i).

[0024] The process combines one of the many processes for enhanced oilrecovery and one of the possible options for the preparation ofhydrocarbons from synthesis gas.

[0025] A major advantage of the process over the prior art is that thecheap and clean nitrogen which is produced in the air separation step inorder to produce oxygen or oxygen enriched air, is now used in theenhanced oil recovery. This results in a more efficient use of theenergy required for the two processes. In addition less capital isneeded. Please note that oil and gas fields are often found in eachother neighbourhood. For instance in Nigeria and the Middle East manyoil and/or gas fields have been found close to each other. Other regionsshow the similar patterns. Converting the gas into liquid hydrocarbonsin the way as described above, results in a nitrogen stream which nowcan be used for the enhanced oil recovery of the adjacent oil fields.Thus, disadvantages of other processes and/or the transport of nitrogenover long distances are overcome. As no additional energy is requiredfor the production of the nitrogen, less environmentally unfriendlycarbon dioxide is produced. It is observed that up till now the nitrogenproduced in an air separation unit is usually vented to the atmosphere.This is at least partly due to the fact that especially gas-to-liquidsplants (using so called stranded gas) are usually at remote locations,far away from industrial activities which could use the nitrogen. Uptill now no suggestion has been made to use the nitrogen for enhancedoil recovery, while there are sufficient locations at which the nitrogenproduced in the air separation unit could be used for enhanced oilrecovery. This is the more remarkable as several suggestions has beenmade as to the use other side products from gas-to-liquids plants, asenergy and water.

[0026] The separation of the oxygen/nitrogen mixture is suitably carriedout according to the cryogenic process as described above. Theseprocesses are commercially available, and well known to the man skilledin the art. The oxygen/nitrogen mixture used in step (i) is preferablyair. Suitably, the stream enriched in oxygen contains at least 50 mol %,more suitably 85 mol % oxygen, based on the total stream, preferably 95mol %, more preferably 98 mol %. Suitably the oxygen depleted streamcontains at least 95 mol % nitrogen based on the total stream,preferably 98 mol %, more preferably 99 mol %. The oxygen depletedstream contains at most 2 mol % oxygen based on the total stream,preferably at most 1 mol %, more preferably at most 0.2 mol%. Ifdesired, all traces of oxygen may be removed.

[0027] The hydrocarbonaceous feed to be used in the present process issuitably methane, natural gas, associated gas or a mixture of C₁₋₄hydrocarbons, preferably associated gas, more preferably associated gasat a remote location. Other possible hydrocarbonaceous feedstocks arecoal, brown coal, peat, heavy hydrocarbons, e.g. crude oil residues,e.g. pitch, and asphaltenes, and bio fuel, e.g. wood, organic wasteproducts and vegetable oils.

[0028] The partial oxidation may be carried out in an oxidation orgasification reactor. A well known process for the partial oxidation ofa hydrocarbonaceous feed is the Shell Gasification Process in which thehydro-carbonaceous feed is partially combusted in a non-catalyticprocess at elevated temperature and pressure. In another embodiment theoxidation is carried out in the presence of a catalyst. Such catalystsare well known in the art and usually comprise one or more noble GroupVIII metals. Steam and/or carbon dioxide may be added to thehydrocarbonaceous feed stream in order to adjust the H₂/CO ratio. Theoxidation is suitably carried out at temperatures between 900 and 1500°C., preferably 1000 to 1350° C., and a pressure between 5 and 120 bar,especially between 25 and 70 bar. Typically the gaseous mixture has anH₂/CO ratio between 1:1 and 3:1, preferably about 2:1. Prior tocontacting the gaseous mixture with a catalyst for the conversion ofthis gaseous mixture into liquid hydrocarbons, it is preferred to removecompounds which could adversely effect the catalyst. In this respectreference is made to the removal of sulphur containing compounds andnitrogen containing compounds (e.g. NH₃ and HCN).

[0029] The purified gaseous mixture, comprising predominantly hydrogenand carbon monoxide, is contacted with a catalyst in the catalyticconversion stage, by which these compounds are converted into liquidhydrocarbons. These liquid hydrocarbons may comprise paraffinichydrocarbons, methanol, aromatic hydrocarbons and the like.

[0030] The catalysts used for the catalytic conversion of the mixturecomprising hydrogen and carbon monoxide into especially paraffinichydrocarbons are known in the art and are usually referred to asFischer-Tropsch catalysts. Catalysts for use in this process frequentlycomprise, as the catalytically active component, a metal from Group VIIIof the Periodic Table of Elements. Particular catalytically activemetals include ruthenium, iron, cobalt and nickel. Cobalt is a preferredcatalytically active metal. As discussed before, preferredhydro-carbonaceous feeds are natural gas or associated gas. As thesefeedstocks usually results in synthesis gas having H₂/CO ratio's ofabout 2, cobalt is a very good Fischer-Tropsch catalyst as the userratio for this type of catalysts is also about 2.

[0031] The catalytically active metal is preferably supported on aporous carrier. The porous carrier may be selected from any of thesuitable refractory metal oxides or silicates or combinations thereofknown in the art. Particular examples of preferred porous carriersinclude silica, alumina, titania, zirconia, ceria, gallia and mixturesthereof, especially silica, alumina and titania.

[0032] The amount of catalytically active metal on the carrier ispreferably in the range of from 3 to 300 pbw per 100 pbw of carriermaterial, more preferably from 10 to 80 pbw, especially from 20 to 60pbw.

[0033] If desired, the catalyst may also comprise one or more metals ormetal oxides as promoters. Suitable metal oxide promoters may beselected from Groups IIA, IIIB, IVB, VB and VIB of the Periodic Table ofElements, or the actinides and lanthanides. In particular, oxides ofmagnesium, calcium, strontium, barium, scandium, yttrium, lanthanum,cerium, titanium, zirconium, hafnium, thorium, uranium, vanadium,chromium and manganese are very suitable promoters. Particularlypreferred metal oxide promoters for the catalyst used to prepare thewaxes for use in the present invention are manganese and zirconiumoxide. Suitable metal promoters may be selected from Groups VIIB or VIIIof the Periodic Table. Rhenium and Group VIII noble metals areparticularly suitable, with platinum and palladium being especiallypreferred. The amount of promoter present in the catalyst is suitably inthe range of from 0.01 to 100 pbw, preferably 0.1 to 40, more preferably1 to 20 pbw, per 100 pbw of carrier. The most preferred promoters areselected from vanadium, manganese, rhenium, zirconium and platinum.

[0034] The catalytically active metal and the promoter, if present, maybe deposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination. The effect of the calcinationtreatment is to remove crystal water, to decompose volatiledecomposition products and to convert organic and inorganic compounds totheir respective oxides. After calcination, the resulting catalyst maybe activated by contacting the catalyst with hydrogen or ahydrogen-containing gas, typically at temperatures of about 200 to 350°C. Other processes for the preparation of Fischer-Tropsch catalystscomprise kneading/mulling, often followed by extrusion,drying/calcination and activation.

[0035] The catalytic conversion process may be performed underconventional synthesis conditions known in the art. Typically, thecatalytic conversion may be effected at a temperature in the range offrom 150 to 300° C., preferably from 180 to 260° C. Typical totalpressures for the catalytic conversion process are in the range of from1 to 200 bar absolute, more preferably from 10 to 70 bar absolute. Inthe catalytic conversion process especially more than 75 wt % of C₅+,preferably more than 85 wt % C₅+hydrocarbons are formed. Depending onthe catalyst and the conversion conditions, the amount of heavy wax(C₂₀+) may be up to 60 wt %, sometimes up to 70 wt %, and sometimes evenup till 85 wt %. Preferably a cobalt catalyst is used, a low H₂/CO ratiois used (especially 1.7, or even lower) and a low temperature is used(190-230° C.). To avoid any coke formation, it is preferred to use anH₂/CO ratio of at least 0.3. It is especially preferred to carry out theFischer-Tropsch reaction under such conditions that the SF-alpha value,for the obtained products having at least 20 carbon atoms, is at least0.925, preferably at least 0.935, more preferably at least 0.945, evenmore preferably at least 0.955. Preferably the Fischer-Tropschhydrocarbons stream comprises at least 35 wt % C₃₀+, preferably 40 wt %,more preferably 50 wt %.

[0036] Preferably, a Fischer-Tropsch catalyst is used, which yieldssubstantial quantities of paraffins, more preferably substantiallyunbranched paraffins. A most suitable catalyst for this purpose is acobalt-containing Fischer-Tropsch catalyst. Such catalysts are describedin the literature, see e.g. AU 698392 and WO 99/34917.

[0037] The Fischer-Tropsch process may be a slurry FT process or a fixedbed FT process, especially a multitubular fixed bed, preferably a threephase fluidised bed process.

[0038] Alternatively the gaseous mixture may be converted, using abifunctional catalyst, in a first step into acyclic hydrocarbons and/oracyclic oxygen containing hydrocarbons. The product formed is at leastpartially converted in a second step into aromatic hydrocarbons suitableas high octane gasoline and chemical intermediates.

[0039] Suitable bifunctional catalysts are known in the art. Thesecatalysts comprise a first catalyst component having activity for theconversion of synthesis gas into acyclic hydrocarbons and/or acyclicoxygen-containing hydrocarbons, such as methanol and dimethyl ether, incombination with a second catalyst component having activity for theconversion of the acyclic compounds mentioned into aromatichydrocarbons. This first catalyst component comprises at least one metalfrom the iron group or ruthenium together with one or more promoters toincrease the activity and/or selectivity, and sometimes a carriermaterial such as kieselguhr, such as catalysts containing from 30 to 75parts by weight iron, and from 5 to 40 parts by weight magnesium per 100parts by weight alumina. The preparation of the catalyst is similar tothat of Fischer-Tropsch catalysts mentioned above. Other suitablecatalysts are ZnO/Cr₂O₃ compositions, in particular such compositions inwhich the atomic percentage of zinc, based on the sum of zinc andchromium, is at least 60%, and preferably from 60 to 80%. Details of thecatalyst and process may be found in the art, for example U.S. Pat. No.4,338,089.

[0040] Suitable examples of the second catalyst component for theproduction of aromatic hydrocarbons are crystalline silicates, forinstance crystalline aluminium silicates (zeolites), crystalline ironsilicates and crystalline gallium silicates.

[0041] The reaction may be carried out at temperatures of from 200 to500°C., preferably from 250 to 450° C., at a pressure of from 1 to 150bar, preferably 5 to 100 bar, and a GHSV between 50 and 5000 N1/1/h,preferably 300 to 3000 N1/1/h.

[0042] Alternatively, the gaseous mixture comprising hydrogen and carbonmonoxide may serve as a starting material for the preparation ofmethanol, using any suitable catalytic methanol synthesis process. Inthis invention methanol is also considered a liquid hydrocarbon.

[0043] The recovery of oil from a subsurface reservoir using the oxygendepleted stream as obtained in the first step of the present process iswell known to the man skilled in the art. In this respect reference ismade to the discussion above concerning the enhanced oil recovery usingnitrogen and the references cited. Suitably, nitrogen, optionally incombination with carbon dioxide and/or steam (individually or inselected mixtures) is injected down-hole at controlled temperature andpressure into the formation. Such injections may be continuously withrecovery at a production well spaced therefrom, or cyclic with recoveryat the injected well (“huff and puff”). The actual requirements willvary from field to field. Temperatures may vary from 20° C. (formiscible light crudes) to 450° C., or even to 600° C., for very heavycrudes. Pressures must be in excess of the formation pressure, usually 2to 50 bar in excess.

[0044] The process of the present invention is suitably combined withfurther additional oil production techniques. In this respect is apreferred embodiment a process in which the oxygen depleted stream ismixed with carbon dioxide. This carbon dioxide is especially produced inthe process in which the synthesis gas obtained in step (ii) isconverted into liquid hydrocarbons. Carbon dioxide may be present in oneor more (recycle) streams from which it may be extracted, or may beobtained by burning certain waste streams as Fischer-Tropsch off gas. Inanother preferred embodiment of the invention, light hydrocarbons aremixed with the nitrogen stream. These light hydrocarbons, suitably C₁ toC₄ hydrocarbons, especially methane, may (at least partly) have beenproduced in the hydrocarbon synthesis reaction.

1. A process for the recovery of oil from a subsurface reservoir incombination with the production of liquid hydrocarbons from ahydrocarbonaceous stream, comprising: (i) separating an oxygen/nitrogenmixture into a stream enriched in oxygen and an oxygen depleted stream;(ii) partially oxidating the hydrocarbonaceous feed at elevatedtemperature and pressure using enriched oxygen produced in step (i) toproduce synthesis gas; (iii) converting synthesis gas obtained in step(ii) into liquid hydrocarbons; and (iv) recovering oil from a subsurfacereservoir using at least part of the oxygen depleted gas stream producedin step (i).
 2. The process of claim 1, in which the oxygen/nitrogenmixture used in step (i) is air.
 3. The process of claim 1, in which thestream enriched in oxygen comprises at least 85 mol % oxygen based onthe total stream.
 4. The process of claim 1, in which the oxygendepleted stream comprises at least 95 mol % nitrogen based on the totalstream.
 5. The process of claim 4, in which the oxygen depleted streamcomprises at most 2 mol % oxygen based on the total stream.
 6. Theprocess of claims 1, in which the hydrocarbonaceous feed is selectedfrom the group consisting of: methane, natural gas, associated gas and amixture of C₁₋₄ hydrocarbons.
 7. The process of claim 6, in which theassociated gas is associated gas at a remote location.
 8. The process ofclaim 1, further comprising additional oil production techniques.
 9. Theprocess of claim 1, in which the oxygen depleted stream is mixed withcarbon dioxide.
 10. The process of claim 9, in which the carbon dioxideis produced in the overall process in which the synthesis gas obtainedin step (ii) is converted into liquid hydrocarbons.